Dark trace storage tube system

ABSTRACT

An electro-optical signal storing device is disclosed and more particularly, an improved &#39;&#39;&#39;&#39;dark-trace&#39;&#39;&#39;&#39; cathode ray storage tube. A cathode ray tube structure is provided having a cathodochromic viewing screen with a thin phosphor layer on its back surface. An electron gun produces an electron beam which is modulated by an input signal. Voltage is applied to the accelerating anode of the tube to allow the beam to penetrate the phosphor layer and darken the cathodochromic screen in the regions determined by the input signal and therefore produces a pattern on the screen. During the initial writing mode, the screen is scanned with a relatively low beam current and a high writing speed, thereby producing a dark-trace pattern having minimal contrast. After the dark trace pattern is written, the input signal is cut off and the anode voltage is reduced. By scanning the screen in raster fashion, the electron beam generates a flying light spot on the phosphor layer which is partially penetrated, causing limited darkening of the cathodochromic screen. During the scanning operation, the light from the phosphor screen which passes through the cathodochromic screen is reflected from a partially transmissive mirror and is sensed by a photocell capable of intercepting light over a wide angle. The output of the photocell is amplified and fed back to the control grid of the cathode ray tube. When the photocell senses a decrease in light, the beam current is increased so that when the beam scans a darkened area, the reduced light produces an increased beam current through the feedback loop such that, during scanning, the previously darkened areas will be further darkened relative to undarkened areas resulting in a high contrast image.

United States Patent [191 Kazan [451 May 29,1973

[54] DARK TRACE STORAGE TUBE SYSTEM [75] Inventor: Benjamin Kazan,

Bedford Hills,

[73] Assignee: International Business Machines Corporation, Armonk, NY.

22 Filed: Aug. 27, 1971 21 Appl.No.: 175,550

Primary Examiner-Reuben Epstein Attarney.lohn J. Goodwin et al.

[5 7] ABSTRACT An electro-optical signal storing device is disclosed and more particularly, an improved dark-trace" cathode ray storage tube. A cathode ray tube struc- DEFLECTION SIGNAL SOURCE ture is provided having a cathodochromic viewing screen with a thin phosphor layer on its back surface. An electron gun produces an electron beam which is modulated by an input signal. Voltage is applied to the accelerating anode of the tube to allow the beam to penetrate the phosphor layer and darken the cathodochromic screen in the regions determined by the input signal and therefore produces a pattern on the screen. During the initial writing mode, the screen is scanned with a relatively low beam current and a high writing speed, thereby producing a dark-trace pattern having'minimal contrast. After the dark trace pattern is written, the input signal is cut off and the anode voltage is reduced. By scanning the screen in raster fashion, the electron beam generates a flying light spot on the phosphor layer which is partially penetrated, causing limited darkening of the cathodochromic screen. During the scanning operation, the light from the phosphor screen which passes through the cathodochromic screen is reflected from a partially transmissive mirror and is sensed by a photocell capable of intercepting light over a wide angle. The output of the photocell is amplified and fed back to the control grid of the cathode ray tube. When the photocell senses a decrease in light, the beam current is increased so that when the beam scans a darkened area, the reduced light produces an increased beam current through the feedback loop such that, during scanning, the previously darkened areas will be further darkened relative to undarkened areas resulting in a high contrast image.

6 Claims, 1 Drawing Figure Patented May 29, 1973 3,736,461

28 SIGNAL SOURCE INVENTOR BENJAMIN KAZAN ATTORNEY DARK TRACE STORAGE TUBE SYSTEM FIELD OF THE INVENTION The present invention relates to electro-optical signal storing and reproducing systems and more particularly to systems employing dark-trace cathode ray tubes.

DESCRIPTION OF THE PRIOR ART Dark-trace cathode ray tubes are known in the prior art. An example of such a device is provided in US. Pat. No. 2,535,817 issued Dec. 26, 1950 to Albert M. Skellett and assigned to the National Union Radio Corporation. The present invention is distinct from prior art devices in that it provides a feedback signal which is used to obtain higher image contrast.

SUMMARY OF THE INVENTION An object of the present invention is to provide an improved dark-trace cathode ray tube.

Another object of the present invention is to provide a dark-trace cathode ray tube including a feedback path to improve image contrast.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawing.

DESCRIPTION OF THE EMBODIMENT The figure is a partial block diagram and partial schematic diagram of a dark-trace cathode ray tube system employing a feedback path according to the principles of the present invention.

Referring to the drawing, a typical dark-trace cathode ray tube envelope is shown including a conventional electron gun comprising an electron-emitting cathode 12, a control grid 14 for controlling beam intensity, a focussing electrode 16 and an accelerating electrode 18. Deflection coils 20 are provided and are connected to a source of conventional x, y deflection signals 21. The face of the cathode ray tube contains a cathodochromic screen 22 which consists, for example, of hackmanite (i.e., cathodochromic sodalite). The back of screen 22 is coated with a thin phosphor layer 24.

In a first mode, referred to as the writing mode, the electron beam is deflected to successive portions of the screen in accordance with input information under the control of the deflection means 20. An input signal from signal source 26 is connected via switch 30 to a control means such as the control grid 14 to simultaneously vary the intensity of the electron beam to produce a desired pattern on the screen 22. The input signal may alternatively be connectedto cathode l2. During the writing of the pattern on screen 22, a sufficient voltage, for example 20 kilovolts from voltage source 32, is maintained on accelerating electrode 18, allowing the electron beam to penetrate the phosphor layer 24 and darken screen 22 to produce a visible pattern in accordance with the input signal. In the writing of the pattern as described, it is sufficient to produce a dark-trace pattern with minimal contrast (for example 5 percent or less) thereby allowing the screen to be scanned with low beam current and at a high writing speed.

The system in the drawing also includes a feedback loop comprising a partially reflective, partially transmissive mirror 34, a photocell 36, capable of accepting light over a wide angle, a switch 38 and an amplifier 40. The feedback loop is connected via a capacitor 42 to a control means such as the control grid 14. During the initial writing mode, switch 38 is in the open position. In the second mode, referred to as the signal intensification sequence, switch 38 is closed and the potential on the accelerating electrode 18 is reduced, for example, to 10 kilovolts by moving a switch 44 from source 32 to voltage source 45. Switch 30 is opened and the electron beam of the cathode ray tube scans screen 22 in a raster fashion without any input signal modulation, generating a flying light spot on the short persistence phosphor screen 24. Due to the lower voltage on the accelerating electrode 18, the penetration of the electron beam into cathodochromic screen 22 is reduced, causing only limited darkening action.

During the signal intensification mode, the light from phosphor screen 24 which passes through cathodochromic screen 22 is modulated in accordance with the previous darkening of the cathodochromic screen. This light is reflected by mirror 34 and detected by photocell 36 which produces an output signal which varies in accordance with the light intensity reaching it. The output signal from photocell 36 passes through closed switch 38 and amplifier 40 and is fed back to control grid 14. The output phase of amplifier 40 is such that when photocell 36 detects a decrease in light, the output signal therefrom is increased, thereby producing a greater beam current. In this manner, previously darkened portions of screen 22 are darkened further relative to the other areas of the screen and a higher contrast display is provided.

During the second or signal intensification mode, when the original recorded signal contrast is enhanced, the electron beam scanning speed may be reduced, for example, to one frame per second or lower so that the frequency response of amplifier 40 need only be kilohertz or less.

To separate the effects of ambient light from the time-varying light signals produced at the photocell from the viewing screen during signal intensification, it may be desirable to chop the electron beam at a high frequency well above the feedback signal frequencies by applying a high frequency carrier signal to cathode 12. In this case, the light from the screen reaching the photocell will consist of an amplitude-modulated highfrequency carrier. By incorporating a narrow-band filter at the input of amplifier 40 which only passes frequencies close to the carrier frequency, and a demodu lating circuit at the output of the amplifier, a feedback signal of proper phase can be generated in the control grid as described previously.

Thus, another embodiment of the present invention may be obtained by modifying the system shown in FIG. 1 by applying a high frequency carrier signal to cathode 12, a conventional narrow band filter connected before the input of amplifier 40, and a conventional demodulating circuit connected at the output of amplifier 40. This produces a new combination but it is not felt necessary to show the combination in a separate drawing since one skilled in the art is capable of understanding the combination from the aforesaid description.

Also, instead of applying a carrier signal to cathode 12, a mechanical light chopper may be placed in front of photocell 36.

An advantage of the present invention is that the resultant written pattern may be viewed for long periods in high ambient light levels, a feature frequently not possible in conventional dark-trace cathode ray tubes because of bleaching which normally occurs during viewing unless very low writing speeds are used to heavily darken the screen.

In typical dark-trace tubes whose screen consists, for example of photochromic sodalite or KCl, it is found that if the integrated amount of beam current reaching the screen is less than about 1 microjoule/cm, the contrast ratio of the dark trace pattern produced is limited, being for example less than 1.5:1. With such low contrast ratios, if the screen is illuminated under normal viewing conditions, the trace disappears rapidly, for example in a period of the order of seconds. It is found, however, that if the total charge deposited on the screen is in excess of microjouleslcm for example, not only is the resulting contrast ratio much greater, but in addition, darkened areas show essentially no decay irrespective of the intensity of the viewing light or its duration, erasing occurring only by applying heat to the screen.

In accordance with the present invention, as a result of fast writing, an initial image with low contrast ratio is produced which persists for a limited time. However, by means of the feedback scanning action described, the contrast ratio of the image is increased and its subsequent decay prevented.

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

l. A dark-trace cathode ray tube system comprising:

a dark-trace cathode ray tube including an electron gun for producing an electron beam, means for controlling the intensity of said electron beam, and a cathodochromic screen for displaying a pattern in response to said electron beam;

a signal source connected to said control means of said dark-trace tube for varying the intensity of said electron beam to produce a dark trace pattern on said screen,

and feedback means responsive to said dark-trace pattern and connected to said control means for producing a feedback signal to said control grid for enhancing the contrast of said dark trace pattern,

wherein said dark-trace cathode ray tube includes a layer of phosphor material proximate to said cathodochromic screen.

2. A dark-trace cathode ray tube system according to claim 1 wherein said feedback means further includes a photocell for detecting variations in the light intensity on the screen of said cathode ray tube and converting said light variations into a representative electrical signal,

and amplifying means for connecting the output signal from said photocell to said control grid of said dark-trace electron tube.

3. A dark-trace cathode ray tube system according to claim 2 wherein said feedback means includes a switch for providing an open circuit condition in said feedback means when said dark trace pattern is being produced on said screen of said cathode ray tube,

and for providing a closed feedback circuit after said dark trace pattern is on said screen.

4. A dark-trace cathode ray tube system according to claim 2 further including a source of high frequency carrier signal connected to said electron gun to chop said electron beam at a frequency higher than the feedback signal frequency to provide an amplitude modulated high frequency carrier signal,

and wherein said feedback means further includes a narrow band filter connected at the input to amplifying means to pass frequencies close only to said carrier frequency,

and a demodulating circuit connected at the output of said amplifier to provide a feedback signal to said control grid.

5. A dark-trace cathode ray tube system according to claim 4 wherein said feedback means further includes an optical chopping means located in front of said photocell to chop said electron beam at a frequency higher than the feedback signal frequency to provide an amplitude modulated high frequency carrier signal,

a narrow band filter connected at the input to amplifying means to pass frequencies close only to said carrier frequency,

and a demodulating circuit connected at the output of said amplifier to provide a feedback signal to said control grid.

6. A dark-trace cathode ray tube system comprising:

a dark-trace cathode ray tube including an electron gun for producing an electron beam, a control grid for controlling the intensity of said electron beam, a cathodochromic screen for displaying a pattern in response to said electron beam, and a layer of phosphor material proximate to said cathodochromic screen;

said cathode ray tube system having a writing mode of operation including an input signal source connected to said control grid by a switch which varies the intensity of said electron beam to produce a dark trace pattern on said screen;

said cathode ray tube system having a second scanning mode subsequent to said writing mode wherein said switch from said input signal source is disconnected and wherein a feedback means is included having means responsive to said displayed pattern for producing a feedback signal inversely proportional to the light intensity of said patterns,

and means for connecting the output signal from said responsive means to said control grid of said cathode ray tube for enhancing the displayed pattern. 

1. A dark-trace cathode ray tube system comprising: a dark-trace cathode ray tube including an electron gun for producing an electron beam, means for controlling the intensity of said electron beam, and a cathodochromic screen for displaying a pattern in response to said electron beam; a signal source connected to said control means of said darktrace tube for varying the intensity of said electron beam to produce a dark trace pattern on said screen, and feedback means responsive to said dark-trace pattern and connected to said control means for producing a feedback signal to said control grid for enhancing the contrast of said dark trace pattern, wherein said dark-trace cathode ray tube includes a layer of phosphor material proximate to said cathodochromic screen.
 2. A dark-trace cathode ray tube system according to claim 1 wherein said feedback means further includes a photocell for detecting variations in the light intensity on the screen of said cathode ray tube and converting said light variations into a representative electrical signal, and amplifying means for connecting the output signal from said photocell to said control grid of said dark-trace electron tube.
 3. A dark-trace cathode ray tube system according to claim 2 wherein said feedback meaNs includes a switch for providing an open circuit condition in said feedback means when said dark trace pattern is being produced on said screen of said cathode ray tube, and for providing a closed feedback circuit after said dark trace pattern is on said screen.
 4. A dark-trace cathode ray tube system according to claim 2 further including a source of high frequency carrier signal connected to said electron gun to chop said electron beam at a frequency higher than the feedback signal frequency to provide an amplitude modulated high frequency carrier signal, and wherein said feedback means further includes a narrow band filter connected at the input to amplifying means to pass frequencies close only to said carrier frequency, and a demodulating circuit connected at the output of said amplifier to provide a feedback signal to said control grid.
 5. A dark-trace cathode ray tube system according to claim 4 wherein said feedback means further includes an optical chopping means located in front of said photocell to chop said electron beam at a frequency higher than the feedback signal frequency to provide an amplitude modulated high frequency carrier signal, a narrow band filter connected at the input to amplifying means to pass frequencies close only to said carrier frequency, and a demodulating circuit connected at the output of said amplifier to provide a feedback signal to said control grid.
 6. A dark-trace cathode ray tube system comprising: a dark-trace cathode ray tube including an electron gun for producing an electron beam, a control grid for controlling the intensity of said electron beam, a cathodochromic screen for displaying a pattern in response to said electron beam, and a layer of phosphor material proximate to said cathodochromic screen; said cathode ray tube system having a writing mode of operation including an input signal source connected to said control grid by a switch which varies the intensity of said electron beam to produce a dark trace pattern on said screen; said cathode ray tube system having a second scanning mode subsequent to said writing mode wherein said switch from said input signal source is disconnected and wherein a feedback means is included having means responsive to said displayed pattern for producing a feedback signal inversely proportional to the light intensity of said patterns, and means for connecting the output signal from said responsive means to said control grid of said cathode ray tube for enhancing the displayed pattern. 